The development and differentiation of mammalian motor units is the result of interactions between muscle cells, motoneurons and the nerve fibers supplying synaptic input to the motorneurons. The early postnatal period in the rat finds all components of this system undergoing developmental changes. Interactions between the components will eventually result in the differentiation of motor units into one of three functional and morphological subgroups. The mechanisms underlying this induced differentiation, however, are unknown. It is possible that motoneuron discharge patterns provide one induction mechanism, and these patterns, in turn, depend on the synaptic organization of motoneuron input. My major hypothesis states that because of the apparent association between motoneuron activity patterns and muscle cell differentiation, reflex development may coincide with motor unit development. In other words, as reflex pathways mature and motoneuron discharge patterns progress toward their adult state, motor unit differentiation will occur concomitantly. I propose to test this hypothesis by first investigating the developmental sequence of reflex maturation and motor unit differentiation in the normal rat. Subsequently, I will investigate differentiation of motor units permanently deprived of one reflex pathway.